1 . Field of the invention
The present invention relates to a pneumatic radial tire comprising an annular tread portion, a pair of sidewalls that extend from both ends of the tread portion in the radial direction of the tire, a pair of bead portions that are provided at the ends of the sidewalls to engage a wheel rim on which the tire is to be mounted in use, a carcass layer that extends through the tread portion and the sidewalls to engage the bead portions, and breaker layers that are arranged in the tread portion in the external radial part of the carcass layer.
2. Description of Related Art
Radial tires are far superior to bias tires in many aspects including fuel economy, life (endurance) and maneuverability, and today radial tires dominate the market of pneumatic tires.
A pneumatic radial tire comprises an annular tread portion, a pair of sidewalls that extend from both ends of the tread portion in the radial direction of the tire, a pair of bead portions that are provided at the ends of the sidewalls to engage a wheel rim on which the tire is to be mounted in use, a carcass layer that is composed of cords that are arranged in the radial direction of the tire as they are engaged at both ends with the bead portions and which extend through both sidewall portions and the tread portion, and beaker layers of high rigidity that are arranged in the external radial part of the carcass layer.
The carcass of radial passenger tires which are generally required to have high maneuverability is composed of two plies in order to provide enhanced lateral rigidity. As recent models of vehicles are designed to run with higher fuel efficiency, the requirements for lighter tires have become more rigorous than ever before. Using a smaller number of plies in the carcass is effective in reducing a substantial portion of the tire weight, for example, a changeover from two-ply to one-ply carcass is effective in reducing a substantial portion of the tire weight but, at the same time, the lateral rigidity of the tire is lowered to impair its maneuverability, in particular, its cornering performance.